This invention generally relates to silica scale inhibitors. More specifically, this invention relates to a method for inhibiting the formation and deposition of silica and silicate compounds in water systems with water-soluble polyamides.
In many parts of the world, amorphous silica scales cause significant fouling problems when industrial waters contain high quantities of silica. For the most part, high quantities of silica means that the industrial waters contain at least 5 ppm and up to about 500 ppm dissolved silica and may contain higher quantities of silica either in dissolved, dispersed or colloidal forms.
The solubility of silica adversely limits the efficient use of water in industrial applications, such as cooling, boiler, geothermal, reverse osmosis and papermaking. Specifically, water treatment operations are limited because the solubility of silica at about 150 ppm can be exceeded when minerals are concentrated during processing. This can result in the precipitation and deposition of amorphous silica and silicates with consequential loss of equipment efficiency. Moreover, the accumulation of silica on internal surfaces of water treatment equipment, such as boilers, cooling, and purification systems, reduces heat transfer and fluid flow through heat exchange tubes and membranes.
Once the silica scale forms on water treatment equipment, the removal of such scale is very difficult and costly. With high silica water, therefore, cooling and reverse osmosis systems typically operate at low water-use efficiency to assure that the solubility of silica is not exceeded. Under these conditions, however, reverse osmosis systems must limit their pure water recovery rate and cooling systems must limit water recycling. In both cases, water discharge volumes are large.
Various additives have been employed over the years to inhibit silica deposition. The current technologies for silica scale control in industrial cooling systems involve the use of either colloidal silica dispersants or silica polymerization inhibitors. Dispersant technologies have shown little activity, being able to stabilize only slight increases of total silica in a tower. For instance, by feeding a dispersant, silica levels may increase from 150-200 to 180-220 ppm, which is often an undetectable increase in silica cycles.
On the other hand, silica polymerization inhibitors have shown to be more effective against silica scale deposition. For example, U.S. Pat. No. 4,532,047 to Dubin relates to the use of a water-soluble low molecular weight polypolar organic compound for inhibiting amorphous silica scale formation on surfaces in contact with industrial waters. Likewise, U.S. Pat. No. 5,658,465 to Nicholas et al relates to the use of polyoxazoline as a silica scale inhibition technology. These polymerization inhibitors have allowed for increases in soluble silica to greater than 300 ppm without scale formation.
This invention provides an improved method for inhibiting the formation and deposition of silica and silicate compounds in water systems. The inventors have surprisingly discovered that certain polymers containing secondary amine and/or ether and amide functional groups are effective inhibitors of soluble silica polymerization and scale development in water systems. The inventors believe that the use of a polymer with such secondary functional groups prevents potential steric hindrance that occurs in prior polymeric formulations.
Accordingly, in its principal aspect, this invention is directed to a method for inhibiting the formation and deposition of silica and silicate compounds in water systems comprising adding to the water in the water system an effective amount of a water-soluble condensation polymer obtained by the polymerization of:
i) at least one carbonyl compound of formula 
xe2x80x83wherein
R1 is selected from linear, cyclic or branched C1-C8 alkylene groups, aromatic groups, polycyclic groups and heterocyclic groups; Z and Zxe2x80x2 are independently selected from halogen, xe2x80x94OH, xe2x80x94OR2, and OC(O)R2; and R2 is selected from linear, cyclic or branched C1-C8 alkyl groups, aromatic groups, polycyclic groups or heterocyclic groups, with
ii) at least one polyamine having at least two amine groups.
An advantage of this invention is it provides a liquid polymeric scale inhibitor that is much easier to handle and feed than the borate-based silica scale inhibitors.
Another advantage of this invention is that it provides a condensation polymer that is less expensive to manufacture than the polyoxazoline proposed in U.S. Pat. No. 5,658,465, resulting in increased cost performance. Moreover, the polymer of this invention is more cost-effective; it has been shown to outperform prior polymeric compositions on an equal polymer actives weight basis.
Moreover, an advantage of the invention is that it minimizes potential steric problems associated with prior polyamide silica inhibitors by utilizing secondary amine and/or ether and amide functionality. Specifically, the polyaminoamides and polyetherarnides of this invention incorporate both the amine (or ether) and amide functionalities into the backbone of the polymer. Therefore, the ability of these functional groups to interact in solution with silica and silicate species is enhanced. On the other hand, U.S. Pat. No. 5,658,465 teaches the use of polyamides created by polymerizing oxazoline monomers. These polyamides contain only amide functional groups, rather than a combination of amide, amine and/or ether functional groups. In addition, the polyoxazolines taught by Nicholas et al have the amide functional groups pendent to the polymer backbone, which the inventors believe causes the activity of these polymers to suffer due to the pendent amide being trapped between the terminal amide R-group and the polymer backbone, which sterically hinders interactions with the silica and silicate species.
Still further, another unique aspect of the polyamide technology of this invention is that it incorporates the activity of the amine and/or ether functional groups as well as that of the amide. In this way, there are more functional sites on the polymer for enhancing the interactions with silica and silicate species in solution as compared to previous polyoxazoline technologies.
The condensation polymers of this invention may be used alone or in combination with other water treating agents. For example, the polymers may be used in combination with phosphoric acids and their salts, phosphonic acids and their salts, metal chelating agents, corrosion inhibitors, polymer scale control dispersants, microbiocides, flocculants, coagulants, oxygen scavengers, neutralizing amines, scale inhibitors, homo- and copolymers of acrylic acid, and homo- and copolymers of maleic acid or anhydride and acrylic acid/maleic acid based polymers.
Additional features and advantages of this invention are described in, and will be apparent from, the detailed description of the presently preferred embodiments.